The Equine Heart: Power Plant Unequaled

The photo on the left shows the outside of a horse’s heart. The photo on the right shows the inside. Ao=aorta; PA=pulmonary artery; LA=left atrium; LV=left ventricle; RV=right ventricle; VM=mitral valve; AoV=aortic valve

Photo: UC Davis Center for Equine Health Horse Report

Reprinted fromThe Horse Reportwith permission from the Center for Equine Health, School of Veterinary Medicine, University of California, Davis (UC Davis).

For as long as horses have been bred, raised, and used by humans, their owners and caretakers have praised their "heart." This reputation for quality or quantity of "heart" is based on a horse's psychological rather than anatomical characteristics: stoicism, bravery, dedication, and nobility of character. But in fact, these attributes of character also reflect the typically robust nature of the horse's anatomical heart and cardiovascular system as well.

A horse’s heart supplies blood to all parts of one of the largest domesticated land mammals (often 1,000 pounds or more), not only at rest and during routine activities, but also during periods of extreme physical stress such as that encountered during performing, racing and the training required for those activities. It is a power plant unequaled in any other creature. That it is able to do so attests to the system’s remarkable efficiency and adaptability.

The cardiovascular system of a horse consists of a pump (the heart), a distribution system (arteries), exchange areas (capillary beds), and a collection and return system (veins). The heart of a 1,000-pound adult horse is about the size of a large melon and weighs about 10 pounds. As in all mammals, it consists of the left and right atrial receiving chambers, left and right atrioventricular inflow valves, left and right pumping ventricles, and aortic and pulmonic semilunar outflow valves, plus attached inflow veins and outflow arteries (see photos above).

The left heart collects blood returning from the lungs via the pulmonary veins and pumps it to the body via the aorta. The right heart collects blood returning from the body via the large veins and directs it to the lungs via the pulmonary artery. The output of each ventricle in an adult horse at rest is about 25 to 40 liters/minute (approximately 7 to 10 gallons), compared with four to five liters/minute in an average adult human (1 liter equals about 1.05 quart).

This Horse Report describes some conditions most often encountered by equine cardiologists, including some common congenital defects, some causes of heart problems, and the diagnostic methods that are currently used. In general, however, horses have excellent cardiac health.

Cardiac Disorders

Horses generally do not have the same types of cardiac problems experienced by humans, such as coronary heart disease (atherosclerosis) and heart attacks. But because most horses are not kept as companion pets and are expected to perform work or athletic feats with a rider, the consequence of any kind of cardiovascular disease in a horse could be greater than it might be in a dog or a cat. In addition to the potential effect of a cardiac problem on performance, the safety of the rider must also be considered in determining the future of the horse.

The main observable signs of a heart problem in horses include:

Loss of condition;

Increased fatigue during exertion;

Shortness of breath;

Increased rate or effort of breathing;

Weakness occasionally resulting in collapse or fainting; and

Signs of fluid accumulation in the abdomen or beneath the skin of the lower thorax.

Depending on the severity of the problem, these signs might initially appear only when the horse is subjected to moderate or strenuous exercise. With a severe cardiac disorder, these signs could appear during normal non-strenuous activities or even at rest.

A physical examination in a horse with cardiac disease will usually reveal an abnormality in the audible heart sounds or in the heart rate and rhythm. When an abnormality is suspected, further evaluations by electrocardiogram (EKG) and echocardiogram (ultrasound imaging) might be performed to more precisely identify the nature and severity of the abnormality. Chest radiographs could also be used to help determine heart size and abnormalities in the lungs and chest cavity, but these are often difficult to obtain with conventional equipment in adult horses because of their large body size.

Heart Murmurs

During an examination of a horse’s chest and heart with a stethoscope, a veterinarian might detect an abnormality by the sound of a heart murmur, which is the sound of turbulent blood flow usually caused by an abrupt increase in the velocity of blood flow. When blood moves smoothly through the heart and blood vessels, very little sound is produced (like water flowing smoothly through a hose).

Most heart murmurs are caused by blood flow that becomes turbulent because of increased velocity due to a leak or obstruction in one of the heart valves or because of abnormal communication between different parts of the heart (like the increased velocity and spraying sound you get when you put your thumb across the end of the hose). However, there are some soft, short, variable heart murmurs that can be heard with no other detectable evidence of heart disease. Such murmurs are referred to as normal or “innocent.”

If there is uncertainty about the origin or significance of any heart murmur, further evaluation is usually performed by echocardiography, which provides detailed images of the inside of the heart and can detect abnormal blood flow patterns.

Grade 2—Readily audible murmur that is softer than S1 (the first of the two beats in a heartbeat) or S2 (the second of the two beats in a heartbeat).

Grade 3—Readily audible murmur that is moderately loud and similar in volume to S1 and S2.

Grade 4—Readily audible murmur that radiates widely and is louder than S1 or S2.

Grade 5—Very loud murmur with a palpable thrill (vibration) that is detectable with fingertip pressure over the heart.

Grade 6—Very loud murmur associated with a palpable thrill that is audible with the stethoscope held just off the chest.

Heart murmurs are further classified by when in the cardiac cycle they occur—during ventricular filling (diastole, after S2 and before S1) or during ventricular contraction (systole, between S1 and S2). Finally, they are described by their length and musical qualities.

The majority of heart murmurs heard in the horse are physiologic or benign. These murmurs can increase in intensity with submaximal exercise and can also be heard with high vagal tone or when the horse is in an excited state. One classic example of this is colic. Often during a painful colic episode a murmur can be heard with a stethoscope that has not been heard before and that will resolve when the colic resolves.

It is important to know which heart murmurs merit further diagnostics and which heart murmurs can be considered incidental. In general, heart murmurs should be assessed by a specialist, especially when accompanied by other signs of cardiac dysfunction or illness. In addition to listening to the murmur with a stethoscope, a cardiologist will be able to assess the heart via echocardiography (cardiac ultrasound). Blood flow through the heart can be analyzed through color flow echocardiography, pinpointing the cause or causes of the murmur. Additionally, the whole heart can be scanned to assess chamber size and contractility.

Congenital Heart Defects

Congenital heart defects are abnormalities that are present at birth. They occur much more common in humans and dogs than in horses. These abnormalities are often discovered within the first few weeks to months of life when a heart murmur is heard during stethoscopic examination of the chest and heart.

Although rare, congenital heart defects can prevent the development of an athletic career and in some cases can be life-threatening. For this reason, every newborn foal should receive a thorough cardiac examination.

A wide variety of simple or complex congenital heart defects can occur, but only a few have been recognized often enough to be reported in more than a few individual horses. The most accurate technique for identifying specific defects and evaluating their severity is two-dimensional echocardiography (ultrasound imaging) supplemented by Doppler echocardiography (imaging of blood flow within the heart and associated blood vessels).

The most commonly reported congenital heart defect in horses is ventricular septal defect, described below. Another defect, patent ductus arteriosus, which is common in humans and dogs, is relatively rare in horses beyond one to two weeks of age.

The ductus arteriosus—a large blood vessel in the fetus that connects the pulmonary artery with the aorta, bypassing the lungs—closes more slowly after birth in horses than in humans and dogs, so that a soft, continuous murmur can be heard in newborn foals up to about a week of age. Horse owners should not panic if their veterinarian pronounces that their newborn foal has a heart murmur, although a follow-up exam should be conducted to make sure the murmur disappears.

Ventricular Septal Defect

Image: UC Davis Center for Equine Health Horse Report

Ventricular septal defects consist of a hole in the muscular wall between the two ventricles and are the most commonly recognized congenital heart defect in horses. It also occurs as one part of more complex defects.

In simple cases, the hole results in the passage of oxygen-rich blood from the higher pressure left ventricle to the lower pressure right ventricle and pulmonary artery, primarily during ventricular systole. Because some of this blood bypasses the lungs, it is not fully oxygenated. A systolic heart murmur is usually heard on the right side of the chest over the cranial part of the heart.

Depending on the size of the hole and the amount of blood passing through it, the pulmonary arteries and veins and the left atrium and ventricle are subjected to an increased workload because of this extra volume of blood. If the hole and the resulting shunt are small, the adverse effect on cardiac function might be minimal, and the horse might be fully capable of engaging safely in moderate physical activities without evidence of fatigue or shortness of breath. If the hole is larger and the shunt is greater, there might be signs of cardiac insufficiency with minimal exertion, and the horse could be very limited in its athletic ability. The nature of the defect can usually be confirmed using two-dimensional and Doppler echocardiography.

Patent Ductus Arteriosus

The ductus arteriosus is a large blood vessel connecting the fetal pulmonary artery to the descending aorta, allowing blood from the right ventricle to bypass the non-functioning lungs and be directed toward the abdomen and placenta. In all mammals, the ductus constricts at or shortly after birth, eliminating this fetal connection and allowing for the normal development of the blood vessels in the lungs.

Unlike most other domestic animal species, persistent slight opening of the ductus arteriosus is quite common in newborn foals. Because the pressure in the aorta is higher than that in the pulmonary artery throughout the cardiac cycle, blood flows through the ductus from the aorta to the pulmonary artery, and a “continuous” murmur can be heard over the pulmonary artery on the left side of the chest.

Closure of the ductus usually occurs within the first week of life and the murmur disappears. If the ductus remains open beyond the first week, it is called a persistent or patent ductus arteriosus. The resulting shunt might cause blood volume overload in the pulmonary arteries and veins and the left atrium and ventricle. Although slight patency (an open state) in the first week is very common in foals, patency beyond the first week is rare.

Complex Defects

Congenital heart defects are uncommon in horses, but when they occur, multiple or complex defects appear to be more common than in other species such as dogs and cats. These can occur as combinations of embryologically unrelated defects, or as recognized combinations such as tetralogy of Fallot (consisting of a ventricularseptal defect, pulmonic stenosis, rightward malpositioning of the origin of the aorta, and right ventricular hypertrophy/thickening) or truncus arteriosus (consisting of a ventricular septal defect and a single large arterial trunk exiting both ventricles).

In the most severe cases, there could be shunting of oxygen-poor, darker venous blood from the right heart chambers to the left heart, bypassing the lungs and causing cyanosis (a bluish color to the membranes of the mouth and eyes) at rest or during exertion. These defects can be diagnosed accurately only by X ray angiography or, more recently, two-dimensional and Doppler echocardiography.

Heart surgery is rarely performed in horses. Congenital heart defects can now be accurately diagnosed using sophisticated ultrasound imaging, but treatment options are very limited for the types of defects horses tend to get.

Acquired Heart Disease

Generally speaking, acquired heart disease is relatively uncommon in horses, although it is encountered slightly more frequently than congenital heart defects. It occurs most often in horses older than five years and only occasionally in younger horses. Degenerative changes affecting the heart valves, myocardium (heart muscle), and lungs are associated with aging, increasing in frequency with age.

The most commonly diagnosed conditions are heart rhythm irregularities and leaks in one or more heart valves. The most common signs associated with heart disease include a reduction in exercise capacity (exertional fatigue), shortness of breath especially following exertion, or the detection of a heart murmur, irregular heartbeat, or other audible abnormality in a horse without other signs of illness. Identification of the electrical rhythm of the heart requires recording of an electrocardiogram.

The normal resting heart rhythm of horses is usually slow (28 to 48 beats per minute) and regular (called sinus rhythm). Many horses also have short pauses in their resting heart rhythm caused by “dropped beats” (called second degree AV block). These are considered to be normal if they disappear during exercise.

Atrial Fibrillation

Atrial fibrillation is an electrical disorder of the heart rhythm—also known as an arrhythmia. There are different kinds of arrhythmias, but the most commonly recognized one associated with diminished athletic performance or more serious signs of cardiac insufficiency is atrial fibrillation. With this arrhythmia, the normally regular organized atrial electrical waves become irregular, disorganized, and chaotic, and the atria fail to contract normally. This results in a very unpredictable irregular heartbeat.

Although atrial fibrillation often develops in horses with advanced structural heart disease and atrial dilation, horses most often develop this arrhythmia with minimal or no detectable additional signs of heart disease. Draft breeds are more commonly affected. In such cases, signs of cardiac insufficiency are usually not recognized at rest or with mild to moderate exertion, but become apparent at more strenuous levels of exercise.

Accurate diagnosis of arrhythmias requires evaluation of an electrocardiogram, where the lack of normal atrial waves and the very irregular ventricular waves can be readily identified. Further evaluation of the structure and mechanical function of the heart by echocardiography is also recommended because the prognosis for treatment, recovery, and return to previous activity levels is directly related to the presence or absence of underlying mechanical cardiac dysfunction.

If there is little or no evidence of underlying cardiac dysfunction, administration of oral or injectable drugs, especially quinidine, is often successful at converting the arrhythmia to a normal rhythm. Most of these horses are able to return to their previous levels of activity and performance, although some experience one or more recurrences of the arrhythmia, necessitating retreatment or retirement from strenuous activity. Quinidine can become toxic to horses at higher dosages and occasionally treatment must be suspended before conversion of the rhythm can occur. A newer catheter option for cardioconversion exists for cases that do not respond to quinidine treatment. This option requires general anesthesia and specialized equipment and is still in the developmental stages.

If serious cardiac disease with atrial dilation is present, the prognosis for functional recovery is poor and conversion of the arrhythmia is usually unsuccessful or temporary. Treatment of the signs of cardiac insufficiency with drugs such as digitalis and diuretics can be considered in selected cases where little physical activity is expected.

Valvular Heart Disease

The most commonly recognized acquired structural heart disorders in horses are degenerative valvular deformities. The process causes thickening and deformity of valve leaflets. These defects result in incompetence and insufficiency of one or more heart valves, associated heart murmurs, and dilation of the chambers that must handle the extra regurgitated blood on either side of the incompetent valve. If the valve leak is severe enough, pressure in the veins leading to the affected side of the heart increases to the point where fluid accumulation (edema) occurs.

Valvular disease is initially diagnosed by the detection of a heart murmur during a physical examination. It is very important, however, to understand that “innocent” murmurs are often heard in normal foals and adult horses. In order to advise an owner or rider about the significance of any heart murmur, it is critical to distinguish between a normal murmur and a pathologic murmur of valvular regurgitation, and to assess the severity of any suspected valve leaks. Two-dimensional and Doppler echocardiography are the most accurate and least invasive methods to help make such determinations.

In general, mild to moderate valvular insufficiency in a horse without reported signs of illness is compatible with continued use for mild to moderate physical activity. More severe valvular disease, especially when it is accompanied by obvious signs of cardiac insufficiency, atrial fibrillation, or severe enlargement of the heart, is cause for a poor prognosis and a strong recommendation against any riding or forced physical activity.

Myocardial Disease

Myocarditis is occasionally suspected in a horse that develops an arrhythmia or other electrical disorder following an infectious disease such as strangles, influenza, or an internal abscess. Toxic damage to the heart muscle can also rarely occur as a result of severe dietary deficiency of vitamin E and selenium, or as a result of ingesting the chemical monensin (usually from cattle feed).

Vascular Disease

Horses are known to develop several types of disorders that affect primarily their blood vessels. However, atherosclerosis—vascular disease associated with high blood pressure, high cholesterol, and fats in humans—is exceptionally rare in domestic animals, including horses.

Therefore, the consequences of this condition in humans—including heart attack, stroke and other peripheral arterial disease—are also rare in horses. The only common condition affecting the veins of horses is thrombophlebitis of the jugular vein(s) caused by repeated jugular vein puncture, injection of material outside the vein, or use of a jugular vein catheter. The resulting chemical or physical irritation or infection in or around the vein causes inflammation, swelling, and tenderness, followed by formation of a firm clot in a small portion or long segment of the vein. Treatment involves removing the cause and applying symptomatic treatment for discomfort and any associated infection. Thrombophlebitis can occur in other large veins in the horse.

Several conditions can affect the systemic arteries in horses. The most common condition is parasitic arteritis (inflammation of the walls of the arteries) due to the vascular migration of the larval forms of the intestinal parasite Strongylus vulgaris. The resulting dilation and thrombosis (and potential obstruction) usually occur at the origin of the large arteries to the intestines, although other arteries might be affected. Fortunately, this condition can usually be treated or prevented by an appropriate antiparasitic drug treatment program. The incidence of this problem declined significantly with the introduction of ivermectin dewormer in the 1980s.

The other most commonly recognized arterial disorder is called aorto-iliac thrombosis. In this condition, a clot develops at the point where the abdominal aorta branches toward the hind legs. The resulting restriction of blood flow to the hind limbs can cause signs of lameness, stiffness, weakness, and abnormal gait that develops during exercise and usually disappears at rest. This condition is often progressive and rarely reversible, markedly limiting the athletic uses of an affected horse.

Finally, degenerative changes in the wall of large arteries can weaken a vessel and predispose it to rupture and bleeding. The most commonly reported sites of such rupture are the root of the aorta in stallions and the uterine artery in mares.

Summary

The causes of heart disease in horses are often multiple and difficult to determine in individual cases. There is increasing evidence that genetic background might play a major role in a horse’s susceptibility to developing disease. Some heart muscle disorders have clearly been shown to have a major genetic component, and familial or breed tendencies in some conditions strongly suggest that genetics play a role in these conditions.

Unlike in humans, diet and exercise have not been shown to be factors in heart disease in horses, since horses almost never develop atherosclerotic vascular disease leading to stroke or heart attack. Almost all heart diseases, except the congenital defects, tend to increase with age in horses, just as they do in humans, dogs, and cats.

There are no known preventive strategies to reduce the likelihood of heart disease in horses. However, owners of all animals should avoid inbreeding, which might increase the risk of congenital heart defects. Veterinarians at the University of California, Davis, strongly urge owners not to breed any animals with known congenital defects of any kind, including heart defects. They also caution that it is probably wise to also avoid breeding horses that have developed an acquired heart disorder relatively early in life, as this might indicate an increased susceptibility for that condition in offspring.

Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.